Spannslokket Member
(Spannslokkleddet)
(New name)
(Previously termed: 4d-β-γ, Isotelusskifer, Øvre Isoteluskalk, lsotelus Shale, Upper lsotelus Limestone).
Main Lithologies. Shale (lower part), limestone, siltstone and shale.
Basal and Hypostratotypes. As for formation.
Definition. As defined here the Spannslokket Member comprises two units recognised by earlier workers. The lower one (Isotelus Shale' '4d-β) is a distinct shale unit in the east six to nine metres thick but further west it thins to three to five metres thick and becomes more calcareous. The lower part of the shale is dark in colour; the upper part greenish grey and silty. Pending a detailed study of the whole Skogerholmen Formation, this is simply regarded as the shaly lower part of the Spannslokket Member and as such it differs from the underlying Hovedøya Member.
The remainder of the Spannslokket Member strongly resembles the Hovedøya Member and is an alternating succession of limestones, siltstones and shales although the development of siltstones up to 30cm thick contrasts with the thinner beds in the lower member. The limestones are bedded in the lower part of this portion of the Spannslokket Member but become nodular in the uppermost part of the member where siltstones become much rarer (Fig. 23). Limestone nodules in the upper part may be only 2-3cm thick and thus resemble the lower part of the Grimsøya Formation but they are not as tightly packed as in the densely nodular part of that formation.
The Spannslokket Member is 20.58m thick in its type development on Skogerholmen and approximately 27.4m thick on Hovedøya As with the underlying Hovedøya Member, this westward thinning conforms to the thickness estimates made by Brøgger (1887 pp.30-31).
Fauna and Age. Although much of the Spannslokket Member is unfossiliferous, some limestone horizons are locally richly fossiliferous, yielding a shelly fauna of Ashgill age which includes trilobites (Owen 1980a, 1981), cephalopods (Strand 1933) and brachiopods. Brøgger (1887 p.30) recorded indeterminate graptolites from the lower shale beds of the member. Chondrites and Planolites burrows are a common feature of some horizons in the member.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.